It was 12:18 a.m. Sunday morning when Stephan Ulamec, home from a night out at the pub, got a text message. “Hi! We got signal from Philae! Call me back!” read the message from Cinzia Fantinati, operations manager for the European Space Agency (ESA) lander that had been hibernating in the shadows on comet 67P/Churyumov-Gerasimenko since its bumpy touchdown on 12 November 2014. Philae's activity and communication had lasted just 57 hours, as its batteries quickly drained in the dark. But ESA was hopeful the lander would reinitiate contact as the comet neared the sun and its solar panels received more illumination.
Saturday night, ESA mission managers received a precious 300 data packets from Philae in a short 85-second-long transmission—bare-bones information about the health of the lander, discovered Ulamec, Philae’s project manager at the German Aerospace Center near Cologne. There was good news: Even in its relatively dark location, Philae’s solar panels were providing the lander with 24 watts of peak power—more than the minimum of 19 watts required for communications—thanks to nearly 3 hours of illumination each day. It was operating at –35ºC—warmer than the –45ºC necessary to boot the computer. And most surprising, there were more than 8000 data packets still sitting in the memory of the lander’s computer, with records of earlier activity. Taken together, the data suggest that Philae had woken from its 7-month-long slumber a day or two earlier, but had been unable to communicate with Rosetta, the spacecraft that is orbiting comet 67P and serves as Philae’s relay to Earth. “Fantastic, right?” Ulamec says. “It’s healthy, temperature is good, power is sufficient. The only thing we have to work on is the duration of the radio link.”
Reestablishing radio contact, and improving on it, is the first order of business. Nothing from Philae was heard during a communication pass midday on Sunday in Europe. ESA will listen again Sunday evening. After that, engineers will begin to play with Rosetta’s pointing and orbit to try to make stronger and longer lived radio connections between the antennas of the orbiter and lander, Ulamec says.
Once regular contact has been achieved, and engineers better understand the spacecraft’s daily thermal and power cycles, they will consider uploading new science commands that were developed over the past month—commands that can be performed quickly, in 2- or 3-hour-long bursts, and which do not require Philae’s batteries to be charged. “Everything in principle demonstrates that Philae did not suffer from the very low temperatures it faced in the months of hibernation,” says Jean-Pierre Bibring, one of two Philae lead scientists, at the Institute for Space Astrophysics in Orsay, France. “We should be able to resume operations as soon as we get longer communication links.”
The new science sequence would activate instruments step by step, starting with ones that make no movements and that have low power demands (about 5 watts). These include instruments that make measurements of temperatures, magnetic fields, and electrical conductivity of the comet surface. A second group of instruments with medium power demands (about 15 watts) could be activated if engineers were willing to perform them without parallel radio contact. These include the lander’s cameras and radio ranging instrument. Fresh pictures in the new illumination conditions, along with information about the exact distance between Philae and Rosetta, could help pinpoint Philae’s location. Last week, the Rosetta mission said it had found a promising location for the lander’s resting place, in a crevice outside a crater on the "head" of the two-lobed comet.
Finally, if the power supply improves slightly, or if it can be used to charge Philae’s drained batteries, instruments with high power demands (about 25 watts) could be activated. These include two ovens that analyze the elemental chemistry of comet samples. They could be operated in "ambient" mode with lower power constraints, by sniffing whatever is nearby, or by heating up any residual material that found its way into the ovens. But the scientists are eager to try and feed the ovens fresh samples. “It is, I believe, in reach. But we have to understand what the details of the available power are,” says Hermann Böhnhardt, the other lead Philae scientist, at the Max Planck Institute for Solar System Research in Göttingen, Germany. Scientists may even use Philae’s drill again, after rotating the lander by about 30º, Bibring says. But the team is wary of upsetting the lander, which came to rest in a precarious position on its side in November after bouncing around the comet in its microgravity environment.
The overall illumination conditions could improve for another couple months, as comet 67P approaches perihelion on 13 August, the closest point in its orbit around the sun. “Until then, the daily power input should increase,” Böhnhardt says. Bibring is glad to be busy with the lander again. “The dream is not only alive, it’s continuing,” he says. “It’s a great adventure.”